Pushing the valve is possible in many ways, for example by pushing it through the hole in the tube with a screwdriver.

Disassembly the valve

When reassembling, change the spring.

Tuning

Step 1 (optional)

Tuning to vastly increase the muzzle velocity and power of your QB78 air rifle!
Muzzle energy increases from about 25 ft/lbs for a .22 caliber gun.
Muzzle energy increases from about 22 ft/lbs for a .177 caliber gun.

Step 2

Valve dimensions change
You should change the dimensions of the indicated elements - use a lathe or a drill and sandpaper;
the effect is big enough when you are able to put a valve into a tube.